1. Field of the Invention
The present invention generally relates to interference cancellation systems, and more particularly to interference cancellation systems with increased sensitivity and techniques for reducing the amount of losses in an interference cancellation system.
2. Description of the Prior Art
FIG. 1 is a functional diagram of a traditional interference cancellation system 1 having a reference input port 2 for providing a sample of an interfering signal, and a receiver antenna 4 for providing a receive signal on receiver transmission line 6 which couples the receiver antenna to a receiver 7. In order to obtain a sample of the interfering signal, reference input port 2 may be coupled to an auxiliary antenna (not shown), for example, if the receiver and source of the interfering signal are not collocated. Alternatively, the reference input port 2 may be directly coupled to the interfering signal source if the source and receiver are collocated. The receive signal includes a desired signal component plus an interfering signal component.
The traditional interference cancellation system further includes a directional coupler 8 operatively connected to the reference input port 2 in order to provide an interfering signal sample on line 10 to a synchronous detector 12. The interfering signal sample is also provided via line 13 to a signal controller 14 which is operatively connected to the reference input port 2. The traditional interference cancellation system also includes a directional coupler 16 which is operatively connected to the receiver transmission line 6, for providing a sample of the receive signal to the synchronous detector 12 via line 18.
The synchronous detector 12 receives both the interfering signal sample via line 10 and a sample of the receive signal via line 18. Thereafter, the synchronous detector compares the interfering signal sample with the receive signal sample and essentially detects the portion of the receive signal sample (i.e., the interfering signal component) that is coherent with the interfering signal sample. The synchronous detector 12 then provides DC output signals on its output ports which correspond to differences in amplitude and phase between the interfering signal sample and the coherent signal component of the receive signal.
The traditional interference cancellation system also includes amplifiers and/or integrators 20 which are connected to the outputs of the synchronous detector 12 so that the DC output signals will be amplified and/or integrated to effectively create DC control signals, which are provided via lines 21, 22 to the signal controller 14.
The signal controller 14, also commonly known as a vector modulator, is regulated by the DC control signals provided by the synchronous detector 12 and integrator/amplifiers 20 of closed loops. As previously described, a first input of the signal controller is provided with an interfering signal sample from the reference input port 2 via line 13. Additionally, two other inputs of the signal controller receive the control signals from the integrator/amplifiers 20 via lines 21, 22. The output port of the signal controller is operatively coupled to a directional coupler 23. A cancellation signal generated by the signal controller is provided on the signal controller output port to the directional coupler 23. Receiver transmission line 6 is operatively coupled to directional coupler 23 such that the cancellation signal is injected into the receiver transmission line carrying the receive signal. Specifically, the cancellation signal, when injected into receiver transmission line 6, effectively cancels the interfering signal component from the receive signal.
In the conventional interference cancellation system of FIG. 1, the signal controller (or vector modulator) and synchronous detector are typically quadrature devices. Quadrature vector modulators are commonly used in interference cancellation systems to adjust the amplitude and phase of a reference signal (such as on reference port 2) which is then injected as a cancellation signal into the receiver system to eliminate or minimize the effect of an interfering signal component in a received signal.
Typically, quadrature modulators have an insertion loss of at least 6 dB. As shown in FIG. 1A, a typical quadrature vector modulator 14 (i.e., signal controller) includes a quadrature hybrid splitter 25 which splits a reference signal into two components (i.e., a 0.degree. phase and a 90.degree. phase intermediary signal). Each intermediary signal is provided to a respective biphase variable attenuator 26 whose attenuation is controlled by the control signals provided at control port 27. The intermediary signals, attenuated by the variable attenuators, are then recombined in an in-phase combiner 28, which provides the cancellation signal on its outport 29. Typically, each of the quadrature hybrid splitter and in-phase combiner has a 3 dB insertion loss which together contribute 6 dB to the insertion loss of the signal controller. When losses due to circuit imperfections such as resistive losses are included, the total insertion loss of the conventional signal controller is in the range of 8 to 10 dB.
Oftentimes, the reference signal does not possess sufficient power to overcome the high insertion loss of the signal controller. If an amplifier is used to compensate for the loss, the amplifier may inject an unacceptable amount of thermal noise into the receiver transmission line, and may distort the reference signal, which leads to poor interfering signal cancellation performance. Therefore, it is desirable to reduce the insertion losses in the reference path (i.e., between the reference input port and the receiver transmission line).